The disclosure of Japanese Patent Application No. 2002-051439 filed on Feb. 27, 2002, including its specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a valve control apparatus and method for an internal combustion engine. More particularly, the invention relates to a valve control apparatus and method provided with means for changing a valve operating characteristic such as valve opening timing (i.e., valve timing), valve lift amount, and open valve period of one or both of an intake valve and an exhaust valve in each cylinder of an internal combustion engine.
2. Description of the Related Art
A valve control apparatus for an internal combustion engine is known that changes an operating characteristic of one or both of an intake valve and an exhaust valve of the internal combustion engine while it is running, so as to enable constant optimal engine performance regardless of the running state of the engine. One known example of this type of valve control apparatus controls one or more of a valve opening and closing timing (i.e., valve timing), a valve lift amount, and an open valve period, and the like of an intake valve and an exhaust valve according to the operating state of the internal combustion engine.
When changing the valve timing, for example, a method is used that changes a relative rotation phase of a camshaft with respect to a crankshaft using a hydraulic actuator or the like. Further, to change the valve lift amount, the open valve period and the like, various methods are used. One method aligns a plurality of cams having profiles with different cam lift amounts and cam operation angles in the axial direction on a camshaft, and switches the cam that drives the valve by moving the camshaft in the axial direction using a hydraulic actuator. Another method changes the valve lift amount and open valve period by providing a cam having a cam profile with a continuous change in the actuation angle and the like, instead of providing a plurality of cams, and moving the camshaft in the axial direction using a hydraulic actuator.
An example of this type of valve control apparatus is disclosed in Japanese Patent Laid-Open Publication No. 6-159021, for example.
The valve control apparatus in the foregoing publication controls the valve timing of an intake valve to an optimal value according to the operating state of the engine. This valve control apparatus is provided with a hydraulic actuator that rotates the camshaft relative to the crankshaft, and an oil control valve able to supply an oil pressure that actuates the hydraulic actuator in a direction to advance the valve timing and an oil pressure that actuates the hydraulic actuator in a direction to retard the valve timing.
Also, the valve control apparatus in the foregoing publication is provided with a cam position sensor that detects a rotation phase difference between the camshaft and the crankshaft. The valve control apparatus calculates the actual valve timing using the cam position detected by the sensor, obtains the difference between a target valve timing set from the operating state of the engine and the actual valve timing that was calculated, and performs feedback control on the oil control valve based on this difference.
For example, this feedback control is made PID control based on the difference, and the opening of the oil control valve is set as the sum of the difference and the components proportional to an integral value and a derivative value of the difference.
According to the apparatus in the publication, the proportional coefficient (i.e., gain) of each of the components of the PID control is set according to the engine speed. Ordinarily, because the oil pressure supplied to the actuator is supplied by an oil pump that is driven by the engine, the discharge pressure of the pump changes according to the engine speed. Therefore, if the gain of each of the components of the PID control are fixed, the response rate of the control may change according to a change in the pump discharge pressure (i.e., the engine speed). Therefore, because the output of the apparatus and the gain of each of the components of the PID control in the foregoing publication are not fixed, but set according to the engine speed, the pressure and amount of oil supplied to the hydraulic actuator can be controlled based on the ability (i.e., discharge pressure, discharge amount) of the engine driven oil pump. Accordingly, consistently stable valve timing control is able to be performed regardless of the engine speed.
By setting the gain of the PID control according to the engine speed, the apparatus disclosed in the aforementioned publication prevents the operation speed of the hydraulic actuator from decreasing by setting the gain large in the low speed region, in which the discharge pressure and discharge amount of the engine driven oil pump decrease, and prevents overshooting and hunting in the control by setting the gain low when the engine is running at high speeds, for example.
With the apparatus disclosed in Japanese Patent Laid-Open Publication No. 6-159021, however, even though control is performed according to the engine speed, there are times, such as when the oil temperature is low when the engine is cold, when the valve operating characteristic is unable to be controlled appropriately.
At times such as when the engine is running but is cold after starting, the temperature of the operating oil supplied to the hydraulic actuator has not risen sufficiently so the viscosity of that operating oil is high. Accordingly, an increase in flow resistance within the oil passages and an increase in friction resistance of the sliding portions, and the like, reduce the operating speed of the hydraulic actuator, thereby lowering the responsiveness in the control over the valve operating characteristic and narrowing the operating range of the hydraulic actuator.
The apparatus in the aforementioned publication compensates for the decrease in oil pressure and oil amount when the engine is running at low speeds by increasing the control gain. However, hydraulic systems and engine driven oil pumps and the like are ordinarily designed so that the discharge pressure and the discharge amount will not change much when the engine speed changes, so changes in the oil pressure and oil amount due to changes in the engine speed are kept comparatively small. In contrast, there are times when the increase in flow resistance and the increase in friction resistance due to increased oil viscosity at low temperatures may become far greater than the increase in flow resistance and the increase in friction resistance due to a change in the engine speed.
Therefore, when attempting to prevent a decrease in control responsiveness by only increasing the control gain when the oil temperature is low, there is a tendency for the increase in the gain to become quite large, which may result in overshooting or hunting or the like, making control unstable. Also, the deterioration in the control accuracy of the actuator due to the increased oil viscosity cannot be corrected by just increasing the gain. Just increasing the gain when the oil temperature is low results in the control becoming unstable, which in turn results in a delay in reaching the target valve operating characteristic, and the like, which ultimately leads to a deterioration in engine performance at low temperatures and a deterioration in the exhaust gas emissions, and the like.
In view of the foregoing problems, it is an object of the invention to provide a valve control apparatus and method that enables the responsiveness in valve control to be improved without losing stability in the control, even when the engine is cold.
According to a first aspect of the invention, a valve control apparatus is provided which changes a valve operating characteristic of an internal combustion engine, the valve operating characteristic including at least one of a valve timing, a valve lift amount, and an open valve period. The valve control apparatus includes a actuator that changes the valve operating characteristic. This actuator is actuated according to a value of a driving signal that is input thereto. The valve control apparatus also includes a controller that detects an operating characteristic parameter indicative of the valve operating characteristic and outputs a driving signal value according to a difference between an operating characteristic target value according to an operating condition of the engine and the detected parameter value to the actuating means. The controller performs a forced driving operation that repeats an operation for maintaining the driving signal at a predetermined forced driving signal value for a predetermined hold time when the difference is greater than a predetermined value.
That is, according to the first aspect, when the feedback control is performed on the actuator based on a difference between a control target value and an actual value for a valve operating characteristic parameter and that difference is large, the value of the driving signal is not determined based on the size of that difference, as it is with the conventional feedback control. Instead, the driving signal is set to an appropriate value and an operation which maintains that driving signal value at this value (i.e., a forced driving signal value) for a certain period of time is repeatedly performed. That is, the amount of change in the valve operating characteristic is controlled by increasing or decreasing the number of times the operation is repeated.
As described earlier, at times when the viscosity of the operating fluid is high, such as when the engine is cold, in order to obtain good response to the valve operating characteristic, it is necessary to greatly increase the gain of the feedback control. If the control gain is greatly increased and the difference between the control target value and the actual value is large, however, the value of the driving signal also increases accordingly, which may result in overshooting or hunting, which may cause a delay in reaching the target value. According to this invention, because the forced driving operation that intermittently maintains, or holds, the driving signal value at a large value only when the difference is large is performed without the gain of the feedback control being increased, the value of the driving signal returns to a comparatively small value each time the hold time elapses. As a result, it is possible to increase the overall operation speed of the actuating means while minimizing overshooting and hunting.
The forced drive signal value does not need to be a fixed value throughout the forced driving operation. It may be any value as long as it is able to reliably change the valve operating characteristic. Further, the forced driving signal value does not need to be maintained at a fixed value throughout one hold time. It may be a value that changes during one hold time as long as it is within a range of a size able to reliably change the valve operating characteristic.
It is preferable that the forced driving signal value be set to a comparatively large value (e.g., a value which will result in the greatest operating speed of the actuator) able to operate the actuator even when the operating range of the actuator is narrow, such as when the temperature is low.
According to a second aspect of the invention, a valve control method for an internal combustion engine having an actuator that changes a valve operating characteristic is provided. The actuator is actuated according to a value of a driving signal that is input thereto. The valve operating characteristic includes at least one of a valve timing, a valve lift amount, and an open valve period. This control method comprises the steps of: detecting an operating characteristic parameter indicative of the valve operating characteristic; outputting the driving signal value according to a difference between an operating characteristic target value according to an operating condition of the engine and the detected parameter value to the actuator. In this control method, a forced driving operation that repeats an operation to maintain the driving signal at a predetermined forced driving signal value for a predetermined hold time is performed when the difference is greater than a predetermined value.